The present work is a further investigation into the effects of the carbon (C), nitrogen (N)
and niobium (Nb) contents on then fracture properties of the Type 347 stainless steels at 316oC. 9
heats of systematically designed alloys were examined. Through SEM-EDS, TEM and XRD analyses,
two kinds of precipitates, Nb(C,N), CrNbN were identified in the Type 347 steels with a high ratio of
wt% N to wt% C, on the other hand only Nb(C,N)s were found in the Type 347 steels with a low ratio
of wt% N to wt% C. The tearing moduli were decreased in the range of 52~60% as the carbon content
increased from 0.03wt% to 0.05wt%. The tearing moduli were lowered by 52~59% in the alloys with
a high nitrogen. It was deduced from the microstructure analysis results that the coarse Nb-rich
precipitates control the fracture resistance of the Type 347 as they act as the potential sites for the
nucleation of micro-voids.